It is well known in the mining art to utilize a raise type of earth boring drill to form vertical shafts between mining tunnels at different levels. One such type of raise drill is described, for example, in U.S. Pat. No. 3,675,729. The tool is used by first drilling a small pilot hole between the two levels. A drill string is then lowered through the pilot hole from rotary drive equipment at the upper level and a raise drill tool is attached at the lower level to the drill string. The drill is then drawn upwardly while simultaneously being rotated by the drill string, the drill reaming out the hole to the desired diameter. In an exemplary embodiment the pilot hole may be 11 inches in diameter and the raise drill reams out a hole 6 feet in diameter.
The stem of the raise drill, which is slightly smaller in diameter than the pilot hole, acts as a guide for the raise drill. However, the stem is subjected to considerable wear due to abrasive contact with the surrounding earth formation and, at the same time, is subjected to considerable stress both of tension due to the lifting force imparted to the drill, twisting due to the torque applied to the drill, and bending due to unequal loading around the circumference of the drill. Usually a protective layer of hard facing is applied to the drill stem to give added protection against the abrasive action of the surrounding earth materials.
Although it is common to provide such drills with replaceable rotary cutters, generally the stems have not been made readily replaceable. Thus when the stem becomes worn or fails in operation, replacement of the drill tool is required. While the replacement of the stem portion of a raise drill has heretofore been proposed, as in U.S. Pat. Nos. 3,659,659 and 3,750,767, such known designs have relied on welded or threaded joints to transfer the torque and longitudinal forces from the stem to the cutter assembly. The raise drill in U.S. Pat. No. 3,750,767 has a steep taper between the stem and cutter assembly. Special cutting, pulling, or torquing tools are then required to disassemble and remove the stem from the cutter assembly, which tools are not always readily available in the mining tunnels where the equipment is being used. Furthermore such known drills do not provide any means for directing cooling or lubricating fluids through the center of the stem to the cutters in the cutter assembly, which presents a unique problem where the stem is to be made separable from the cutter assembly.